Vacuum tube



Patented Aug. 18, 1942 VACUUM TUBE Edwin c. Ewing, Jr., chicago, 111. Application July 276, 1939, Serial No. 286,527

4 Claims.

,The present invention relates to vacuum tubes such as the well-known arnplier and detector tubes used for the reproduction of sound from minute currents. In such devices, the fundamental construction comprises a heated element or emitter and a plate element which is maintained at a positive potential with respect to the heated element, so that electrons may flowfrom the heated element to the plate. The amplica-V tion is accomplished by controlling the flow of electrons by means of a grid element which is interposed in the path of the electrons and charged with the current to be amplified so as to vary the flow of electrons between the heated element and the plate.

The present invention is applicable to devices of the above character lwhich contain the three basic elements and other elements such as a heater for the emitting element and additional grid elements as are commonly employed. It is to be understood, therefore, that while the device is shown in connection with a simple triode tube, it is not limited to a triode type of amplier. The preferred form of the invention is embodied in a tube such as that shown in my prior copending application for Non-microphonic amplier tube, Serial No. 233,041, filed October 3, 17938.

It is the purpose of this invention `to provide a tube embodying at least the basic elements of a triode tube, with a diode plate for causing a variation in mu of the tube.

It is also the purpose of the present invention to provide in combinationwith the filament of a tube such as that shown in my prior application, a second diode or plate element which acts automatically to control the output of the tube and to provide a varying bias potential which can be applied to preceding tubes in the usual cascade amplifier.

The purposes and advantages of the invention will appear more fullyfrom the following description, referencebeing had to the accompanying drawing, wherein the preferred form of the invention is shown. It is to be understood, however, that the drawing and description are illustrative only and are not to be taken as limiting the invention except insofar as it is limited by the claims.

In the drawing,

Fig. 1 is a sectional view taken vertically through a tube embodying the present invention;

Fig. 2 is a sectional view taken on the line 2 2 of Fig. 1;

Fig. 3 is a sectional view taken on the line 3--3 of Fig. 1; and

Fig. 4 is a diagrammatic view illustrating the circuit connections for a tube embodying the present invention.

Referring now to the drawing, the present invention is embodied in an evacuated tube 5 which may be of any suitable material such as is commonly used for radio tubes. There are five leads into the tube 5, which leads are identified by the numerals 6,1, 8, 9 and II). A spacing element II of insulating vmaterial maintains the spacing of the leads at a distance somewhat above their point of entry into the tube 5. The lead 6 supports a plate I2 which, as shown,- s in the form of a substantially rectangular hollow metal screen. The lead 'I supports a grid I3 which, as shown, is in the form of a spiral wire wound about two spaced posts I4 and I5. The post I5 carries the grid Wire I3, and this wire in turn carries the opposite post I4. The leads 9 and I0 support a filament I6 which is in the form of a loop of inverted U-shap'e. The filament I S is of the type shown in my c o-pending application hereinbefore referred to. This filament has a portion II at the center thereof, which `projects a short distance above the grid plate assembly.

Asecond plate I8` is carried by the lead I9 and extends over the grid and plate assembly in close proximity to the portion II of the lament I6. Theplate I8 is preferably so formed as to provide a support for a piece I9 of getter material. 'Ihe function of the getter material is, of course, the same in this device as in ordinary tubes of this character. The plate I8, however, has a diierent function that will be more fully described hereinafter.

In the wiring diagram shown in Fig. 4, the usual connections for the tube are illustrated, the output of the tube being connected to a suitable device `for` changing electrical impulses into sound. As an example, thereis shown a usual head-phone 20. The .plate I8 is illustrated as connected to 'the output circuit of the plate I2 through a capacity ZI. A lead 22` is also shown to indicate how the potential on the plate I8 may be fed to the input of a preceding tube or tubes of an amplifier.

The operation of the tube will now be described. There is a radial field about plate I8 when this element is charged by an E. M. F. The plate I8 acts as a deflector to the electron stream when it is at negative potential. 'I'he mu of the tube is maximum at this instant. When the plate I8 is made positive, it attracts electrons, thereby drawing them out of the field of their normal triode function and decreasing the mu of the tube. It will be noted that the mid-part of the filament indicated at I'I is very close to the plate I8. There is thus a direct flow of electrons from the filament to the plate I8. It is noticed with this arrangement that a positive peak actually causes the output of the tube, of which the plate I8 is a part, to fall off during the interval of such a peak due to a lowered amplification factor.

The diagram of an automatic volume control circuit (Fig. 4) shows the output tube containing the diode plate. Another and perhaps preferred circuit whereby better advantage can be taken of the variable mu effect would be to incorporate this diode plate in a higher mu tube than is conventional in an output stage. An ideal arrangement would be to couple the output of the power stage to a diode plate contained in the driver tube. This gives a varying mu in the driver tube. This diode plate can then be connected through a suitable network to the grids of preceding tubes (including the grid of the driver tube, if desirable) to give these tubes a varying negative bias for conventional automatic volume control action. Likewise, a proper network (eliminating feedback) can be coupled to diode plates in other preceding tubes thereby getting an automatic volume control action entirely by means of varying mu and without any change of grid bias.

The mostI substantial use, however, for the provision of the diode relation between lament I'I and plate I8, is in applying a variable negative bias to the grids of previous tubes in a multitube amplifier. These positive peaks are effectively shunted and dissipated by the rectifying action of the diode because the electron stream from the filament will conduct all positive charges to the filament and thus to ground potential. However, the negative peaks are left undisturbed, and these, in turn, are coupled to the preceding grids through a network of essentially syllabic constants. With increased input to the diode, the varying negative bias to the preceding tubes is increased, thereby decreasing the amplifier output. The use of a variable negative bias is common in the normal method of automatic volume control operation. The present tube construction, however, is particularly ladvantageous for this purpose. 'Ihe location of the plate I8 in immediate proximity to the portion I'I of the filament is possible only because there is no support hook between the ends of the filament. It is necessary to have strong emission in the portion I 'I of the filament. This could not be obtained where the filament is Vsupported by a top hook.' The hook would cool the part of the filament most needed. Furthermore, the hook would interfere With the close spacing between the filament and the plate I8.

`The foregoing description is believed to'be sufcient to enable those skilled in the art to clearly understand the nature of the present invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A thermionic vacuum tube having a plate and a grid, a filament comprising a generally U-shapcd wire loop supported at its ends, said wire loop extending into the grid, and a plate element, overlying the mid-portion of said filament for receiving emission from the electronically active mid-portion of the filament at the open end of the grid, and a condenser connecting said plate element and the first named plate.

2. A thermionic vacuum tube comprising a filament emitter comprising an inverted loop, supports for said emitter at the ends thereof, a grid and a plate encircling said emitter, the midpoint of the emitter projecting beyond the grid. and a plate electrode in close proximity to said mid-point for receiving emission from the electronically active mid-point of the lament emitter at the open end of the grid, said plate electrode having an external lead adapted to be capacitatively coupled to the plate of another tube.

3. A thermionic vacuum tube comprising a filament emitter in the form of an inverted loop, supports for said emitter at the ends thereof, a grid about said emitter, a plate electrode outside said grid, the mid-point of the inverted loop of the emitter being located at one end of the grid and a second plate electrode at said end of the grid in close proximity to said mid-point whereby to receive emission from the electronically active mid-point of the filament emitter or repel the emission, the second plate electrode being operable to defiect emission from the mid-point of the filament outwardly to the first plate electrode when it is at negative potential and to attract emission when it is at positive potential.

4. A thermionic vacuum tube comprising a filament in the form of a generally U-shaped wire loop supported at its ends, supports for said ends, a grid encircling said filament and a plate electrode outside the grid, the plate electrode and the grid being located between the midpoint of the filament and the supports for the ends thereof, a second plate electrode extending across the ends of the grid and thefirst plate electrode adjacent the mid-point of the filament and being closer to the mid-point of the filament than the first plate electrode whereby it may attract electrons directly from the mid-point of the filament at the end of thegrid when it is of opposite potential to that of the filament, or deflect emission from the mid-point of the filament outwardly to the first named plate electrode when itis of like potential to that of the filament.

EDWIN C. EWING, Jn. 

